   Chapter 2, Problem 91PS

Chapter
Section
Textbook Problem

Mandelic acid is an organic acid composed of carbon (63.15%), hydrogen (5.30%), and oxygen (31.55%). Its molar mass is 152.14 g/mol. Determine the empirical and molecular formulas of the acid.

Interpretation Introduction

Interpretation:

The empirical formula and molecular formula of the mandelic acid is to be determined if the molar mass is 152.14g/mol.

Concept introduction:

• Empirical formula of a compound represents the smallest whole number relative ratio of elements in that compound.
• Equation for finding Molecular formula from the empirical formula,

MolarmassEmpiricalformula mass × Empirical formula

• Equation for number moles from mass and molar mass,

Numberofmoles=MassingramsMolarmass

• Mass percent of elements of a compound is the ratio of mass of element to the mass of whole compound and multiplied with hundred.
Explanation

Mass percent of C, H and O in mandelic acid is given as 63.15%, 5.30% and 31.55%.

Mass percent of an element means, 100g of compound contains that mass percent of an element in grams. Therefore, mass of C, H and O are 63.15g, 5.33g and 31.55g  respectively in 100g of mandelic acid.

Equation for number moles from mass and molar mass is,

Numberofmoles=MassingramsMolarmass

Therefore,

The number of moles of carbon is,

Numberofmoles=63.15g12.01g/mol=5.258mol

The number of moles of hydrogen is,

Numberofmoles=5.33g1.008g/mol=5.258mol

The number of moles of oxygen is,

Numberofmoles=31

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